This invention relates to an apparatus for manufacturing thermoformed plastics trays, such as blister packs.
A blister pack essentially comprises a containing part consisting of a thermoplastic film provided with container cells for containing products of various kinds, mainly pharmaceutical products of the pill, tablet, capsule and similar types. The container cells are closed hermetically by a metal foil generally of aluminium fixed over them by thermowelding.
The usual method for manufacturing blister packs comprises heating the thermoplastic web, forming the container cells, filling the container cells with the product and sealing the cells with the metal foil, then separating the individual packs from the resultant strip.
The apparatus used at present are of two types. In the first type, the thermoplastic web is wound on a rotating cylinder comprising a plurality of cellular cavities connected together, and means for exhausting the air, heat being radiated over an arc of the cylinder by suitable lamps. During rotation, the web, which is kept adhering to the cylinder, is drawn into the underlying cavities by the vacuum existing therein so that it assumes their shape, after which it leaves the cylinder to take up a flat position where it is filled. The filling and sealing stages then take place followed by separation of the packs by a cutting operation which results in a piece of reticular scrap from the old strip.
These and similar apparatuses which utilize vacuum for forming are able to operate only with webs of very limited thickness which sometimes due to the drawing process become thin to the point which makes the tightness of the product container cells precarious.
Furthermore, radiant panels of non-uniform heating effect are usually used for heating. Another disadvantage is due to the fact that the container cell walls, being formed about a cylinder, tend to retain the curvature when they pass to a flat position, so producing difficulties during the subsequent cutting and insertion into their packet.
In other apparatus, the web to be formed is fed intermittently while the heating, forming, filling, sealing and cutting operations take place with the web flat during the feed interruption step, which both limits the possibility of correctly placing the product in the container cells and imposes low productivity on the apparatus as a whole.
In other apparatus, although the web is fed intermittently and is heated and formed when flat and at rest, filling takes place with the strip under continuous uniform motion, with a buffer or store being interposed between the forming and filling station to allow temporary accumulation of the web.
In addition, downstream of the filling station there is provided a second buffer or store to enable the web to again pass from continuous movement to the intermittent movement required by the sealing means.
In this case, in addition to a more complicated production line due to the problems of coupling and interdependence between the various working units, the production line length is considerable and its control by the operating personnel is difficult.
A common characteristic of known apparatus is the production of a notable quantity of scrap in the form of reticular strip.
In known apparatus, the presence of intermediate scrap is also to be seen in relation to the need for centering the spacing of the printing reproduced on the covering strip with the effective pitch of the pack. In fact, these apparatuses are based on the principle of keeping the print spacing larger than the pack pitch so as to easily centre the strip, then shrinking to a greater or lesser extent the excess printed strip in the region between one tray and the other, and then removing a suitable strip thereof together with the lateral scrap.
Another method for centering the print consists of providing a theoretical spacing for the print and varying to a greater or lesser degree the distance between one set of impressions and the next. This system is generally used in machines of reciprocating movement driven hydraulically.